It is common practice to pump lift gas into the annulus between a production tubing and surrounding well casing and to pump the lift gas subsequently into the production tubing from the annulus via one or more one way gas lift flow control devices in side pockets that are distributed along the length of the production tubing. The lift gas which is injected through the flow control devices into the crude oil (or other fluid) stream in the production conduit reduces the density of the fluid column in the production conduit and enhances the crude oil production rate of the well.
Commercially available gas lift flow control devices typically use one way check valves which comprise a ball or hemisphere or cone which is pressed against a valve seating ring by a spring. If the lift gas pressure is higher than the pressure of the crude oil stream in the production conduit then this pressure difference exceeds the forces exerted to the ball by the spring so that the spring is compressed and the ball is lifted, or moved away, from the valve seating ring and lift gas is permitted to flow from the gas filled injection conduit into the production conduit. If however the pressure of the crude oil stream is higher than the lift gas pressure in the injection conduit, the accumulated forces of the spring and the pressure difference across the gas lift flow control device push the ball or hemisphere against the ring shaped seat, thereby closing the check valve and preventing crude oil, or other fluid, to flow from the production conduit into the injection conduit.
A problem with the known check valves is that the ball or hemisphere and ring-shaped valve seat are exposed to the flux of lift gas, which may contain liquids or sand or other abrasive particles and/or corrosive chemical components, such as hydrogen sulfide and carbon dioxide. The ball or hemisphere and valve seat are therefore subject to mechanical and chemical erosion, which may result in leakage of the valve, so that crude oil or other fluids may flow into the injection conduit from the production conduit, and may block further lift gas injection when the crude oil, or other fluid, level in the injection conduit has reached the location of the gas lift flow control device or flow control devices.
U.S. Pat. No. 5,535,828 discloses a surface controlled gas lift valve which is retrievably inserted in a side pocket in the production tubing of an oil well, wherein a frustoconical valve body is mounted on a hydraulically actuated piston which can be actuated from surface to press the valve body against a frustoconical valve seat and to lift the valve body from the valve seat. The valve body and valve seat are exposed to the flux of lift gas and subject to mechanical and chemical erosion.
It is known from U.S. Pat. No. 5,004,007 to provide a surface controlled chemical injection valve, wherein a flapper type valve body and associated ring-shaped valve seat are protected from exposure to the flux of injected chemicals by a protective sleeve that is pushed by hydraulic pressure through the ring-shaped valve seat and which is pushed back by a spring once the hydraulic pressure has decreased below a threshold level, thereby permitting the flapper type valve body to swing against the ring-shaped valve seat. The known chemical injection valve is equipped with a flow restriction connected to the valve housing and a piston, which is actuated by the pressure difference across the flow restriction. The piston is arranged in a cylindrical cavity in the valve housing adjacent to the sleeve and is connected to the sleeve. The piston serves to overcome frictional forces between the sleeve and any seals between the sleeve and valve housing and the presence of the piston adjacent to the sleeve makes the valve complex, expensive and prone to failure if contaminants, sand or abrasive particles accumulate in the cylindrical cavity above the piston, and/or if the seals fail.
The complex design of the surface controlled chemical injection valve renders it unsuitable to replace the known wear prone spring actuated ball valves.